1. Field of the Invention
The invention relates to battery powered computer systems, and more particularly, to circuits and methods for reducing the power consumption of the computer system.
2. Description of the Prior Art
Portable computer systems are rapidly developing the capabilities of conventional desktop or floor mounted personal computer systems. Hard disk units are being integrated into portable computers because of the large amounts of information being processed and the large size of many application programs. A floppy disk unit is integrated in the vast majority of portable computers, even if a hard disk unit is installed, to allow loading of information and use of applications requiring key disks, and also to allow use of diagnostic programs. Modems have been integrated into portable computers for some time to allow communications and information transfer between the user and a remote location, for example, the home office. The displays in portable computer systems are becoming much more elaborate and readable. The pixel count on the standard liquid crystal displays (LCD's) utilized is increasing, as is the viewing angle. The use of backlighting allows use of LCD's in low light environments and improves the contrast ratio of the display. More complex circuitry is being installed in portable computers to support these improved peripheral devices and to support the increased speeds and capabilities of the microprocessors utilized in portable computer systems.
The various peripheral devices and high speed circuitry mentioned above consume large amounts of power when operating. This has resulted in problems in portable computer systems because these systems are generally desired to be used in locations where alternating current is not available. This has made it very difficult to provide all the possible functionality available and yet have an acceptable battery life when the portable computer system is battery powered. Using CMOS components helped reduce the power consumption of the circuitry, but even the use of CMOS components is insufficient at the clock speeds and performance levels of available circuitry. Therefore a dilemma arises whether to provide lesser functionality with longer battery life or greater functionality with lesser battery life or even no battery operation.
Various alternatives were tried to resolve the problem. For example, the International Business Machines (IBM) Corporation PC Convertible included a switch which the user could press to place the computer system in a standby mode. However, the PC convertible was relatively simple, with a low level of functionality as compared to what is currently available, and the requirement of a user action to reduce power consumption limited its use to circumstances where the user remembered to depress the switch. Blanking the display after a period of keyboard inactivity saved power as well as prolonged the life of the display and was widely utilized. A hard disk unit was developed which reduced the power used by the controlling electronics by utilizing only certain portions of the track for servo information and turning off the read channel circuitry until just before a servo burst was expected. Additionally, a programmable value could be provided to the hard disk unit so that after a given inactivity interval defined by this value, the hard disk unit was allowed to spin down and all but some interface circuitry was shut down. While these alternatives did provide some relief, they were not complete solutions to satisfactorily resolve the dilemma, and design tradeoffs still were forced to occur.
U.S. Pat. No. 4,980,836 to Carter et al. discloses an apparatus for reducing power consumption in computer systems. The apparatus monitors the address bus to determine when selected peripheral devices have not been accessed for a preset amount of time. When the preset amount of time has passed, the system powers itself down and disables the system clock, placing the system in a standby mode. The system clock could be stopped in this invention because the preferred embodiment of this invention used a static CMOS processor and chip set. If there was sufficient energy in the batteries, the system could be awakened by the user depressing a standby switch. Computer systems which do not use a static CMOS processor or chip set generally reduce the clock frequency when a preset amount of time of address bus inactivity has passed. Reducing the clock frequency during inactive periods reduces power consumption during this time. However, the frequency of peripheral device accesses is not a completely reliable indicator of inactivity of a computer system. Thus, in some instances the system clocking signal may be reduced in frequency or disabled during a period of high computer system activity. Therefore, a method is needed whereby other elements or events of the computer system can be monitored to more reliably determine the activity level of the computer system so that the system clock can properly be adjusted to reduce power consumption.